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Modern automotive bodies are made of high and ultra high strength steel up to 90 mass percent of the body in white. These steels do not only possess a high light weight potential but also increase crash performance of the passenger cabin. The described benefits are facing the problems of forming these steels. With a low formability at room temperature and high flow stresses the complexity of parts out of these materials is limited. In a warm forming process with temperatures up to 600 °C the formability could be increased and the process forces could be decreased. Especially high complex parts are produced by hydroforming a technology allowing undercuts and uniform strain distributions in the part. For temperatures up to 600 °C there is a need of an appropriate medium for hydroforming. Fluids normally used at room temperature are only temperature stable up to about 350 °C and tend like gases to leakage. Using a granular material like small ceramic beads allow high temperatures and reduce the risk of leakage. The Drucker-PragerCap material model allows to describe this medium for the numerical simulation. The accuracy of the numerical simulation with the gained material model is compared with different experiments in which the parameters were identified and a real part geometry of a cup formed with granular material as medium.
The finite element analysis (FEA) has become one of the most relevant and most important tools in fields of sheet metal forming for designing processes and dimensioning parts. However, reliability and quality of the numerical results strongly depend on the whole FE-model and especially on the modeling of the material behavior, which shows wide impact on calculated stresses and strains of sheet metal parts. Therefore, the experimental determination of characteristic material data concerning anisotropic and temperatureeffects is essential. In this paper the influence of temperature on the yielding and the hardening behavior of the magnesium sheet metal alloy AZ31 are investigated for different uniaxial and biaxial stress conditions. For that purpose an experimental setup has been developed at the Chair of Manufacturing Technology (LFT) which enables biaxial tensile testing of sheet metal. Yield loci of AZ31 are determined as a function of temperature and they are based on solely measurement data of the forming process itself.
The need of light weight construction for high efficient vehicles leads to the use of new materials like aluminium and magnesium alloys or high strength and ultra high strength steels. At elevated temperatures the formability of steel increases as the flow stresses decrease. Forming high complex geometries like chassis components or components of the exhaust system of vehicles can be done by hydroforming. The hydroforming process by oils is limited to temperatures of approximately 300 °C and brings disadvantages of possible leakage and fouling. Using granular material like small ceramic beads as medium could be an approach for hydroforming of ultra high strength steels like MS W1200 and CP W800 at temperatures up to 600 °C.
The material properties of granular material are in some points similar to solid bodies, in other points similar to liquids. For understanding and simulation of the behaviour of the medium a basic characterisation of ceramic beads with different ball diameters is necessary. Powder mechanics and soil engineering give ideas for experimental setups.
For the conversion of these approaches on the one hand the behaviour of the ceramic beads itself has to be characterized, on the other hand the contact between a blank and the beads have to be investigated. For the tests three different kinds of spheres with a diameter between 63 microns and 850 microns are used. In unidirectional compression test compressibility, pressure distribution in compression direction and transversal compression direction and the effect of bead fracture are investigated. The tests are carried out at different compression velocities and for multiple compressions. For determination of friction coefficients between blank and beads and determination of shear stress in bulk under compression a modified Jenike-Shear-Cell for use in universal testing machines with the possibility of hydraulic compression of the beads is built up. The gained data can be used for material modelling in ABAQUS using Mohr-Coulomb or Drucker-Prager model.
Increasing demands regarding security aspects and light weight construction lead to the application of advanced high strength steels (AHSS) and ultra high strength steels (UHSS) in the automotive sector. Due to high process forces and the reduced formability of these steel grades within cold forming new manufacturing technologies like the hot stamping process are required. Furthermore, crash-performance plays an important role in the automotive industry. Therefore functional optimized components are necessary. Hence, actual research work within the community is focused on manufacturing components with local adjusted mechanical properties. One of the strategies to realize the contradictorily requirements regarding energy absorption and structural integrity is the Tailored tempering process where the cooling rates are adjusted by controlled heating or cooling of different tool zones within the hot stamping process. Thereby knowledge concerning the influence of the different heated tool parts on the heat transfer and the resulting mechanical properties is necessary. Furthermore, the applicability and the accuracy of the calculation approaches used for characteristic values like the heat transfer coefficient in the FE-based simulation have to be analyzed and evaluated. Due to this experiments with a tool which exhibits a heated and a cooled zone were performed according to the Tailored tempering process. During the experiments contact pressures and tool temperatures in the heated tool part were varied and analyzed regarding the influence on the heat transfer. Furthermore, the heat transfer coefficients were calculated and verified by a numerical model built according to the experimental setup and the accuracy of the model was evaluated by the comparison of characteristic values calculated from the experimental and numerical process data.
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